Today, image forming apparatuses such as multifunction printing apparatuses (MFP apparatuses) are widely used as one of OA apparatuses related to information. Such image forming apparatuses can easily output (print) image information supplied from a host apparatus, such as a personal computer, connected to LAN. In addition, many image forming apparatuses have a scanner function, and therefore can easily copy a document.
However, there is a problem that such image forming apparatuses undesirably allow easy leakage of important information to an outside, although such image forming apparatuses conveniently allow easy printing and copying of image information. In view of this, various devices and methods for preventing leakage of information that occurs through printing and copying have been conventionally proposed.
For example, Patent Literature 1 discloses a technique of forming, on a recording sheet, an image created by adding an additional information image formed with the use of an invisible toner to an image formed with the use of a visible toner. The invisible toner is a toner which can hardly be observed under visible light, and is, for example, a toner containing a fluorescent substance that glows when irradiated by ultraviolet light.
As long as the additional information image is formed on a printed material with the use of an invisible toner, it is possible to judge whether the printed material is the original one or not by checking presence or absence of the additional information image. That is, a document on which the additional information image is formed is the original one, and a document on which the additional information image cannot be found is a copy (false document) of the original one. Moreover, even in a case where an original document is falsified, and then the document thus falsified is copied and even in a case where a digitalized original document is falsified, and then the document thus falsified is printed, an additional information image is removed from such documents. Accordingly, even in these cases, it is possible to judge whether a document is a falsified one or not by checking presence or absence of an additional information image.